Mixing grid

Abstract

A grid for use in nuclear fuel assemblies is made up of double grid members which are deformed so as to provide channels for the coolant. At least the upper portions of these channels are inclined relative to the axes of the fuel rods and are disposed in such a manner as to cause circulation of fluid about each individual rod and within the assembly. In one embodiment the grid is positioned between the positioning grids and serves primarily as a coolant deflector. In another embodiment so constructed as to serve the positioning grid, as well as a flow deflector, the channels are straight in their lower portions i.e. parallel to the axes of the fuel rods. In their upper portions they curve to provide the desired deflection of the cooling fluid and the straight sections have a slight bow outward, relative to the plane of the grid member of which they are a part. This outwardly bowed portion engages the fuel rod and acts as a positioning spring. The grid members are cut away adjacent the straight sections to give the necessary flexibility. This design is particularly useful in pressurized water reactors. In grids intended for use in boiling water reactors, where the lateral loading is less severe, the straight portion can be omitted. The two plates may be welded together at their intersections, at their upper edges only, while the lower edges are left free. In this case, the flexing of the grid members themselves serves to provide the spring forces necessary to position the fuel rods. In order to give improved flexibility, and proper fit with the rods when loaded, the surfaces of the channels which contact the fuel rods are given a twisted or helical formation.

The embodiments of the invention in which a proprietary right or priviledge is claimed are defined as follows: 
     
       1. A grid for a nuclear reactor fuel assembly comprising a plurality of horizontally extending grid strips, said strips being arranged in sets at an angle to each other, forming polygonal spaces between them; said strips being formed with generally vertically extending tubular passages, at least the upper portions of said passages being inclined to the vertical, and being disposed about each of said spaces to produce a swirling motion of water above said spaces. 
     
     
       2. A grid as defined in claim 1, wherein each strip comprises two narrow sheets of metal, each having an upper and a lower edge, at least one of said sheets of a given strip being deformed to give channels inclined to the perpendicular relative to said upper edge, said sheets being positioned with the channels of said one sheet mating with the other sheet to form said passages. 
     
     
       3. A grid as defined in claim 2, wherein each of said sheets is deformed to give said channels. 
     
     
       4. A grid as defined in claim 3, wherein each of said channel comprises a surface facing into one of said polygonal spaces so as to contact a fuel rod, said surface being twisted in such a manner that, when a fuel rod is inserted, it will be twisted toward a flattened configuration. 
     
     
       5. A grid as defined in claim 3, wherein the channels of said sheets are mated to give said passages. 
     
     
       6. A grid as defined in claim 2, wherein the strips of each set intersect the strips of the other set, and wherein the sheets forming each of said strips are welded together at one only of said edges at the intersections, thereby giving a spring action to said strips. 
     
     
       7. A grid as defined in claim 1, wherein each of said passages comprises an upper and a lower portion and wherein the lower portions of said passages are vertical. 
     
     
       8. A grid as defined in claim 2, wherein each of said channels comprises a lower portion, which is substantially perpendicular to said upper edge, and an upper portion which is inclined to the perpendicular relative to said upper edge. 
     
     
       9. A grid as defined in claim 8, wherein said perpendicular portion is bowed outwardly intermediate its length, relative to the plane of the sheet from which it is formed, thereby providing a spring. 
     
     
       10. A grid as difined in claim 9, wherein said grid strips are provided with apertures adjacent said perpendicular portions to impart added flexibility. 
     
     
       11. A grid as defined in claim 10 wherein said apertures are elongated and parallel to said perpendicular portions. 
     
     
       12. A nuclear fuel assembly comprising a large number of closely spaced parallel rods; a plurality of spacer grids positioned at right angles to said rods, each of said grids being formed of at least two sets of narrow, thin plates at an angle to each other, thereby forming polygonal spaces, through which said rods extend, said grids being spaced along said rods;   mixing grids positioned between at least some of said spacer grids, and positioned substantially perpendicularly to said fuel rods;   each of said mixing grids comprising a plurality of grid strips extending perpendicular to said rods, said strips being arranged in sets at an angle to each other, forming polygonal spaces through which said rods extend;   said strips being formed with tubular passages, at least the upper portions of said passages positioned at acute angles to the axes of said rods, and being disposed about saidspaces in such a manner as to produce a swirling motion of water above said spaces.   
     
     
       13. A fuel assembly for a boiling water nuclear reactor comprising a plurality of elongated, parallel, closely spaced fuel rods; at least one spacing and deflecting grid positioned perpendicularly to said rods, said grid comprising at leat two sets of grid strips positioned at an angle to each other and defining polygonal spaces through each of which a fuel rod passes;   each of said grid strips being formed with tubular passages inclined at an acute angle to the axes of said fuel rods and postioned in the walls of at least the major portion of said polygonal spaces;   at least some of said passages being positioned so as to cause swirling of water above one of said spaces about the rod passing through it.   
     
     
       14. A fuel assembly as defined in claim 13 wherein some of said passages are positioned so as to deflect coolant passing through said assembly away from surfaces cooler than said coolant. 
     
     
       15. A fuel assembly as defined in claim 14 wherein one of said surfaces forms the periphery of said assembly and the passages adjacent the polygonal spaces next to the periphery are arranged to direct the coolant inwardly. 
     
     
       16. A fuel assembly as defined in claim 14 wherein at least one of said surfaces is a tube parallel to said fuel rods, through which water flows during reactor operation, and wherein some of said passages are positioned to direct coolant away from said tube.